The present invention relates to large fluid storage tanks and, more particularly, to a method of preventing the bursting of storage tanks made of a material which is subject to failure under load by a rapidly propagating fracture. The present invention also relates to reinforced storage tanks produced in connection with the method.
In 1988, an oil storage tank in Floreffe, Pennsylvania near Pittsburgh burst, spilling about 3.8 million gallons of oil into the Monongahela River. The oil flowed down the Monongahela River and then down the Ohio River causing immense damage. The owner of the tank received more than 4,000 claims totalling $18,000,000 from individuals, businesses, and government agencies, and paid clean-up costs and provided drinking water or assistance to more than 17 communities along the rivers from Pittsburgh to Louisville, Kentucky.
The failure of the tank, which was made of steel and built in the 1940's was due to a dime-size flaw in the tank's steel shell, which was present when the tank was first erected. The fact that the flaw was near a weld caused the material near the flaw tip to become embrittled, and the embrittlement led to a fracture which rapidly propagated from the flaw to the top and the bottom of the tank, probably in less than one second. The embrittlement is believed to have been caused by the phenomenon of dynamic-strain-aging, but other factors can lead to the failure of a tank, such as thermal cycling from the change of seasons, load cycling from the filling and emptying of the tank, and corrosion. The bursting of the 48-foot high, 120 foot diameter tank was so sudden and so complete that: the weld joint attaching the tank shell to the bottom plates failed; the ruptured tank shell, propelled by the force of the gushing oil, moved over 100 feet in a direction opposite the rupture, taking the tank roof with it; and the oil stream hit an adjacent tank with such force that it dented the steel shell.
The failure of the tank has caused concern that other catastrophic failures could occur. The concern is not only that further pollution problems could arise, but that escaping flammable liquids could ignite, causing a tremendous explosion that could involve an entire tank farm. It is estimated that there are presently over 20,000 of these 1930's/1940's vintage liquid storage tanks on the navigable waterways of the United States. Because of its type and age, the steel used in these tanks is subject to embrittlement and rapidly propagating brittle fractures, especially at low temperatures. Thus, the tanks are subject not only to stresses imposed by the fluids contained, but also to additional stresses imposed by flaws in their shells. However, despite the fact that the damage caused by the bursting of one of these tanks is tremendous, the cost of replacing them or emptying and reconstructing them is so great that no extensive activity in replacing or reconstructing them has taken place.
In years since the 1940's, as an alternative to steel, it has been proposed that large storage tanks be constructed of helically-wound filamentary material which is either impregnated with an unpolymerized resin before winding or treated with a subsequently applied resin. An example of such a tank having a shell made only of a composite material of strands of glass fibers and a resin is disclosed in U.S. Pat. No. 2,808,097 to W. G. Martin. The tank is constructed using a mandrel comprising a plurality of arcuate plates which are removed when the forming of the storage structure has been completed. Another example of a shell made of resin and filaments is disclosed in U.S. Pat. No. 3,537,938 to H. R. Clements, wherein the shell includes several layers of resin or resin and filaments in various forms.
It has also been known to form large structures of a plurality of panels made of glass fiber reinforced plastic resin honeycomb bonded together at their abutting edges and to reinforce such structures with strands of filaments coated with the resin and wrapped around the structure, in order that the structure is strong enough to withstand the high radial forces which translate into hoop tension when the structures are used for storing liquids. Such structures, like that disclosed in U.S. Pat. No. 3,819,450 to Bernard P. Kunz, seek to obtain the strength of the conventional steel storage tank without the weight and cost which are associated with steel tanks.
Although proposals have been made to avoid the problems of steel storage tanks by constructing new tanks of composite material, such tanks have been designed merely to withstand the forces of the fluids in the tanks. Thus, the additional stresses due to flaws in materials have not been dealt with, and the problems of making thousands of existing steel tanks safe and constructing safer new tanks of steel have not been solved.
No effective or practical solution has been proposed for the problem of making aging steel storage tanks safe. Since the storage tanks comprise a large number of steel plates welded together, previous approaches to solving the problem of the deteriorating storage tanks have included from time to time, carefully inspecting the welds for defects and correcting any defects found, as well as inspecting the plates for corrosion, repairing any spots where corrosion is found, and, if the corrosion is serious, removing and replacing the particular plates affected. In some instances, repairs have been effected by securing patches over those areas exhibiting defects or flaws. To be successful, these approaches rely on the inspections to find the problems that exist and on the repairs to adequately correct the problems. This is for the reason that patches are only secured in those areas in which defects are found; thus, undetected flaws and defects remain as potential sources of problems.
Other approaches have involved proposals for avoiding the conditions which lead to the failure of storage tanks. In an article in the Oil & Gas Journal of Feb. 19, 1990, entitled "Brittle Fracture of Old Storage Tanks Can Be Prevented", a number of suggestions were made for avoiding such conditions. Since low temperatures lead to failures, it was suggested that external insulation be provided on the tank shell so that the liquid stored in the tank will keep the shell relatively warm. Since high stress at low temperatures can lead to tank failures, the article proposed limiting the fill height of tanks to less than 75 percent of the their shell height during winter conditions (for material having low toughness). The article further suggested determining any sources of shock loading of the tank shell and avoiding them if possible. Since high stress concentrations can lead to failure of the tanks, the article proposed water testing the tanks. In the case of oil storage tanks, because the specific gravity of the liquids to be stored are generally between 0.7 and 0.9, filling the tank with water will test the tank at a significant overload relative to its normal loading in service. The article disclosed that the overload has the beneficial effect of causing locations in the tank shell having high stress concentrations, such as at discontinuities or defects, to yield so that stress peaks are removed and crack tips blunted.
A drawback with water testing is that it must be carried out during warmer weather. It also requires that the tank be emptied of the liquid that it normally stores. A problem with limiting the fill height of the stored liquid in the tank to less than 75 percent of the shell height during winter conditions is, of course, that not as much liquid can be stored. All of the approaches seeking to avoid conditions which lead to the failure of storage tanks have the problem that the tanks are subject to failure if the measures taken are not successful in avoiding the conditions. Furthermore, the condition avoidance remedies such as providing an insulation and limiting fill height, are usually based on the worst conditions expected for a particular site. If conditions which occasionally occur, such as record cold snaps, happen at the tank site, the measures taken may not be adequate to avoid subjecting the tanks to failure.